80 CATALOGUE OF 



p. I. about 1 the length of the hinge-line, narrow, strong, 

 straight ; base strong, slightly swollen ; apex on distal 

 side of centre, prominent; ridges somewhat sharp, 

 descending about equally and fairly gradually. 

 p. in. about I the length of, and parallel to p. /., straight, 

 strong ; apex central, prominent, obtuse-pointed ; ridges 

 somewhat rounded, descending about equally and not 

 steeply. 



L. V. a. //. nearly | the length of the hinge-line, narrow, strong and 

 straight ; base strong, somewhat swollen ; apex central, 

 very prominent, sharply acuminate, slightly dis tally 

 directed ; ridges sharp, descending about equally, very 

 steeply. 



#. not very prominent, subtrigonal, the rounded apex back- 

 wardly and outwardly directed, base continuous with 

 that of a. //. 



4. lamelliform, sharp, arcuate, at first almost parallel with 

 the shell-margin and then curving diagonally backwards 

 half across the hinge-plate. 



p. if. about g the length of the hinge-line, narrow, strong, 

 curving with the shell-margin ; base strong and 

 straight ; apex on distal side of centre, distally directed, 

 very prominent, acuminate; ridges sharp, descending 

 fairly equally and very steeply, especially at first. 



Dimensions. Jenyns' measurements, converted into millimetres, 

 give : Long. 4, Alt. 3, Crass. 2 ! 5 mm. A specimen in Mr. C. Oldham's 

 collection from Cemmaes (Anglesey) is identical. One from 

 Walworth (Londonderry) is 4x3'3x2'6. Malm's is a trifle less, 

 viz.: 3-75x3x2-6 mm. 



As a rule the species is immediately recognizable by its sculpturing. 

 Specimens in which the striae are faint may bear some external 

 resemblance to rounded forms of P. nitidum, but in P. pulchellum 

 the hinge is proportionately narrower, and the outer laterals 

 (a. in. & p. in.} longer, the apices of the inner laterals, especially 

 the anterior ones, are further from the umbo, whilst the cardinals, 

 especially 3 of the right valve, are straighter and more parallel to 

 the hinge-line. 



At Silverdale (Lanes.) a strongly striate form of P. inijtboryii 

 occurs in association with the present species and at first escaped 

 detection (79). Examples of the former are consequently figured 

 (PL XXIII, f . 2 & 18) with the latter for comparison. 



Young forms of P. pulchellum are squarrosc (PL XXIII, f. 11 a & 

 12 a) and the specimens from the Pleistocene of West Wittering, 

 though more adult (PL XXIII, f. 13*), take that form. 



The most oval variant, and the largest seen, comes from Walworth 

 (Co. Londonderry) (PI. XXIII, f. 8, 9 & 16). Other shades of 

 variation in outline and sculpturing are best gathered from the 

 figures. 



